Idea
One identity can contain several versions
An element's identity is set by its number of protons. Add or remove a neutron and that identity remains, although the atom's mass changes. The result can also change nuclear stability. Chemical similarity therefore does not mean two atoms must behave identically in every respect.
Carbon makes the distinction easy to count. Carbon-12 contains six protons and six neutrons; carbon-14 has six protons and eight neutrons. Both remain carbon. The number after the name counts protons plus neutrons, rather than giving the number of protons alone.
Same element does not mean same nucleus.
Separate the property that defines a category from the properties that vary within it.
Where it breaks down
Isotopes are not perfectly identical chemically in every setting: mass differences can affect reaction rates and physical behavior. Nor is every isotope radioactive. The category describes nuclear composition, not a universal instability or hazard.
How it connects
Protons define the category while neutrons distinguish its nuclear forms.
This idea appeared in The same element with a different nucleus, the Involves connection for September 29, 2026, which asked: What can change inside an atom without changing which element it is?
Check yourself
What does the 14 in carbon-14 count?
Protons plus neutrons. Exactly. Mass number counts the nucleons, so carbon-14 has six protons and eight neutrons.
What the sources establish
Carbon-12 and carbon-14 have six protons but six and eight neutrons respectively; isotopes differ in mass and may differ in stability.
Sources
DOE Explains: Isotopes (U.S. Department of Energy), Definition, carbon examples, stability, and mass differences